Various arrangements of optical layouts are known for projection system with reflective liquid crystal displays. Examples are described in U.S. Pat. Nos. 6,309,071 and 6,113,239 and in High Contrast Color Splitting Architecture Using Color Polarization Filters, Michael G. Robinson et al, SID 00 Digest, p. 92-95. One optical layout described in the Robinson et al. article employs proprietary polarization filter technology (i.e., ColorSelect™ polarization filter technology), available from ColorLink, Inc. of Boulder, Colo., USA, to achieve a reported contrast of more than 500:1.
However, contrast in these known systems is limited due to use of MacNeille prisms as the polarization beam splitters (PBSs) in different arrangements. A MacNeille prism PBS has limited contrast due to skew-ray depolarization effects, as described in U.S. Pat. No. 5,327,270. The depolarized light reduces the contrast of reflective electronic projection displays, and particularly those employing liquid crystal-on-silicon (LCOS) light valves. As described in the '270 patent, compensation for the skew-ray depolarization requires an additional quarter-wave plate, which increases cost, requires precision alignment and restricts the range of operating temperatures.
Generally, reflective liquid crystal on silicon (LCOS) light valves have several advantages for use in projection displays, including small pixel size, high aperture ratio, and fast response. As the numerical aperture (NA) of a system using reflective light valves is increased to maximize image brightness, however, contrast decreases. This reduction in contrast is largely due to the interaction between the non-ideal retardance of the light valves and compound angle depolarization by the tilted polarizing beamsplitters (PBSs) typically used in such systems; the contrast varying approximately with the inverse square of the numerical aperture. In addition to reduced contrast, increased NA results in poorer image quality due to increased geometric aberration in the projection lens.
Another limitation of conventional systems is the color temperature or balance of the light. Projection systems typically require a lamp with long lifetime and extremely small source of light, such as is provided by high-pressure mercury lamps (e.g., UHP type, available from Philips Electronics). These lamps produce a discontinuous spectrum and are relatively deficient in one or two primary colors, requiring at least one of the primaries (typically green, and sometimes green and blue) be attenuated to obtain an acceptable white point. This is typically done by limiting that primary to a narrower bandwidth than required to obtain a satisfactory color gamut. For example, the color separation filters are modified to reduce the spectral width of the green and blue primaries, causing them to become more saturated than those specified in the SMPTE broadcast standard and restricting their dynamic range.
Accordingly, the present invention provides high contrast, balanced color and high throughput in a wide variety of electronic projectors, such as a multi-path, reflective liquid crystal-on-silicon (LCOS) projection display system.
In one implementation, a reflective liquid crystal-on-silicon projection system includes an illumination system that generates polychromatic light. A color separation system, such as a cross-dichroic, is positioned to receive the polychromatic light and to separate it into primary color components of light that are directed along separate primary color component light paths. At least one color modifying (e.g., balancing) aperture stop is positioned along at least one of the primary color component light paths to balance relative intensities of the primary color components of light.
A reflective liquid crystal-on-silicon light valve is positioned with a polarizing beam splitter, such as a wire grid polarizing beam splitter, for each of the primary color component light paths to separately impart image information into each of the primary color components of light. A color combiner receives and combines the primary color components of light with imparted image information to provide light representing a polychromatic display image.
In another implementation, a color balancing aperture stop such as an apodizing aperture stop may be positioned to color balance the light before it is color separated. For example, the apodizing aperture stop may include an annular color filter corresponding to the primary color component of light of the primary color component light path in which the apodizing aperture stop is positioned.
The one or more aperture stops provide attenuation by reducing the numerical aperture (or increasing the F-number) of one or more primary color channels. The aperture stops may be implemented in various ways, including use of a smaller illumination system aperture stop, where a separate aperture stop location exists for each primary, or use of a smaller projection lens aperture stop, where separate projection lenses are used for each primary, or use of an annular color filter at the aperture stop of the illumination system or projection lens, where a common illumination system or projection lens is used for all primaries.
Additional description and implementations of the present invention will be apparent from the detailed description of the preferred embodiment thereof, which proceeds with reference to the accompanying drawings.